Heterosis is the effect that progeny obtained through cross-pollination show better agronomical properties than progeny derived through self-pollination. These better properties result in a higher profit which is the reason that the development of hybrid seed is one of the prime objectives for the seed industry.
To obtain hybrid seeds it is necessary that the female plants are unable to self-pollinate. Many crop plants accommodate both the female and male reproductive organs on the same individual and thus self-pollination is dominating cross-pollination. Introduction of male sterility in many crops is thus necessary for hybrid seed production.
In production fields the male-sterile acceptor and male-fertile donor plants are grown side by side and after cross-pollination the hybrid seed is harvested. Hybrid seed is separately collected from the non-hybrid seeds formed on the donor plants by destruction of these male-fertile plants before the harvest. This approach makes the discrimination between fertile and sterile plants necessary which can be accomplished by the appearance of different phenotypes. Also a selectable marker gene such as a gene coding for a herbicide resistance, closely linked to the male-sterility locus can be used for this purpose. Selection of the hybrid seeds can than be accomplished by spraying the herbicide, which will result in the destruction of the male-fertile plants. Alternatively the hybrid seeds can be selected after harvesting by a phenotypic marker expressed at the seed level.
Nowadays in agricultural practice male-sterile parental lines are obtained by physically emasculation of the plants or by the application of cytoplasmic or nuclear encoded male-sterile mutants. There are only a limited number of natural male-sterile mutants available in the commercial interesting crops. This renders the latter approach often not feasible. In addition naturally male-sterile plants have their disadvantages. There preparation is laborious and there maintenance and propagation difficult. They often show additional, detrimental traits and a difficult inheritance.
The development of nuclear encoded male-sterile plants by the application of recombinant DNA techniques has succeeded in new approaches which circumvent most of the disadvantages mentioned above.